The operator of a winch or hoist, either on a crane or on a truck, cannot always see the load that is being moved which can present a hazard when being lowered. When making a lift or pull, operators are frequently guided by an additional person giving directions either by radio or by hand signals. A need exists to prevent the operator of a winch or hoist from spooling off all of the cable on a winch drum and particularly a requirement exists for an indicator of how many wraps of cable remain on the winch drum. In the industry it is typically called a “third wrap indicator” and as the name implies this device warns the operator with an audible or visual signal when only three wraps of cable remain on the winch drum.
Several methods are known to determine the amount of cable that is left on a cable drum of a winch. These frequently use separate idler sheaves that the cable engages which can lead to error due to cable slippage. In another method an algorithm is used to calculate the amount of cable on the drum. The algorithm method considers the revolutions of the drum and the amount of cable that is analytically on the drum at any instant. However, it relies on the assumption that the cable wraps perfectly for each revolution of the drum. Current systems all use an indirect means to calculate the amount of cable that is on the drum or has passed over the drum. As the cable is wound on the drum, each layer has a different amount because the circumference has changed. The current systems rely on the proposition that each wrap of cable is wound analytically perfect. This is very difficult to achieve in practice and therefore it is difficult to accurately measure the length of a cable as it wraps on the drum.
U.S. Pat. No. 3,883,859 shows a load height indication with a rotation-measuring device and indicator to display the load position in numbers of feet from a reference point.
U.S. Pat. No. 4,387,295 shows a pneumatic penetration sensor for oil drilling with paying out of a first layer of cable from the drum, the sensor pulses being coupled to a first pneumatic counter, which is preset so as to provide an output pulse corresponding to a predetermined increment of motion during the paying out of the first layer. Additional counters are provided with different preset limits so that they will provide output pulses corresponding to the predetermined increment of motion during the paying out of second and additional layers of cable from the drum.
U.S. Pat. No. 4,475,163 shows a system for calculating and displaying cable payout from a rotatable drum storage device with up/down counter 43 to count the net value as accumulated from a predetermined reference zero position, e.g. “0” cable paidout. A calculating capability in arithmetic unit 44 performs the required computations and a clock source 45 provides a timing reference for a computer.
The following references are of general interest for showing cable payout from a drum.
U.S. Pat. No. 2,683,020; U.S. Pat. No. 3,750,130; U.S. Pat. No. 4,119,299; U.S. Pat. No. 4,334,217; U.S. Pat. No. 5,906,358; U.S. Pat. No. 5,988,596; DE010029757; DE019900916; GB002074970.